This invention relates to a method of removing plastic film from a substrate, and more particularly, to the removal of an exposed photopolymer film resist from printed circuit boards.
In the manufacture of printed circuits, an epoxy - glass cloth laminate board having a uniform thickness is laminated with or plated with a metal such as copper. Holes are then drilled in a predetermined pattern for accommodating various electrical components. Next, a film of partially polymerized plastic photoresist is deposited on the laminate. These photoresist films may commonly be composed of acrylic resins, polyvinyl cinnamates, diazo compounds, phenol-formaldehydes, polystyrene or other similar film-forming materials. This film is further polymerized, or crosslinked, by the action of ultraviolet light, into a hard chemically resistant film.
By using an appropriate glass or plastic maskant the resist film is selectively hardened in specific predetermined areas, while the resist film in other areas is left unchanged. The unchanged resist film is then dissolved in a "developer" by a solvent such as 1,1,1-trichloroethane or a solution of butoxyethanol and sodium carbonate or similar solutions. The copper in the cleaned areas may then be removed by etching or additional copper and other metals may be plated thereon.
In either event, it then becomes necessary to remove the exposed hardened resist film from the laminate.
It is known that such resist films can be removed by the action of strong organic solvents, such as methylene chloride or trichloroethylene. However, the use of strong organic solvents presents serious problems. They are extremely flammable and exhibit toxic effects when inhaled or contacted. The photoresist dissolves in the organic stripping liquid and eventually reaches a solubility saturation point beyond which the liquid is no longer useful and must be discarded. Since the solvents are not readily soluble in water, serious disposal problems are presented. While it is true that the saturated solution can be distilled to recover the solvent, this requires expensive and sophisticated equipment and controls.
Finally, these solvents are slow acting at room temperature and generally must be heated to a temperature between about 130.degree. - 160.degree. F to give reasonable stripping rates. Of course, increased temperature increases considerably the flammability and toxicity of the solvents and their vapors.
It is also known to use a stripping composition which is made in an aqueous form and usuable only with aqueous developed films. These are not considered suitable for stripping solvent-based resist films. The solutions consist of mixtures of strong alkalies such as sodium hydroxide, and particular organic solvents, such as ethylmonobutyl ether. As in the case of strong organic solvents, the use of these alkaline aqueous stripping solutions also presents the problems of toxicity, special disposal arrangements, and increased cost. Further, as in the case of strong organic solvents, the alkaline aqueous stripping solutions require heating for effective use. The use of higher temperatures increases the problems of flammability and toxicity and in addition, multiplies the corrosive effect of the strong alkaline components.
Printed circuit boards manufactured in accordance with either of the above described methods generally require subsequent treatment to fully remove photoresist residue and both may result in a discoloration of the copper.